This revised resubmission of a six-site follow-up proposal requests funding for a five year study of the safety of methylphenidate (MPH) in children who had been originally recruited and randomized into the preschool Attention-Deficit / Hyperactivity Disorder (ADHD) treatment study (PATS). We propose to continue our multi-site collaboration to conduct a systematic follow-up study of the children who were diagnosed with ADHD in preschool years (ages 3-5 years) and participated in the original Preschoolers with ADHD Treatment Study (PATS) to determine the safety and efficacy of methylphenidate. The proposed follow-up study will assess these children as they turn 4-9 years of age. It will be based at the New York State Psychiatric Institute (NYSPI), Duke University, Johns Hopkins University (JHU), New York University (NYU), University of California at Los Angeles (UCLA), and University of California at Irvine (UCI). The Primary Aim is to address whether early treatment with MPH is associated with emergence of any negative physical (decreased height and weight growth rates), cognitive, or behavioral (e.g., sleep disturbance, tics) effects. The Secondary Aim is to collect information on the course of psychopathology and functioning of the PATS children.